Abstract: A resin composition comprising an organic acid ester of cellulose, as a resin component, and at least one member selected from organic phosphite compounds having the formulae (i) and (ii): ##STR1## wherein R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5 and R.sub.6, which can be the same or different, each is hydrogen, alkyl, aryl, alkylaryl, arylalkyl, alkoxyaryl, alkoxyalkyl, polyalkoxyalkyl, polyalkoxyaryl or polyaryloxyalkyl groups, with the proviso that one of R.sub.1 and R.sub.2 and one or two of R.sub.3, R.sub.4, R.sub.5 and R.sub.6 can be hydrogen, and R.sub.1 ' and R.sub.2 ', which can be the same or different, each is hydrogen or alkyl, aryl, alkoxyalkyl, aryloxyalkyl, alkoxyaryl, arylalkyl, alkylaryl, polyaryloxyalkyl, polyalkoxyalkyl and polyalkoxyaryl groups, and n is a number of at least 1, preferably 1 to 5.